“PowerStager” Tool Employs Unique Obfuscation

A malicious tool that has managed to fly under the radar since April 2017 is showing great focus on obfuscation, in an attempt to evade detection, Palo Alto Networks warns.

Dubbed PowerStager, the tool has shown an uptick in usage for in-the-wild attacks around December 2017. Developed as a Python script that generates Windows executables using C source code, it uses multiple layers of obfuscation to launch PowerShell scripts to execute a shellcode payload.

PowerStager uses a unique obfuscation technique for PowerShell segments, while also offering increased flexibility, due to multiple configuration options.

Some of these options include the ability to target both x86 and x64 platforms, support for additional obfuscation on top of defaults, support for customized error messages/executable icon for social engineering, and the ability to use Meterpreter or other built-in shellcode payloads. The tool can also fetch remote payloads or embed them into the executable and can escalate privileges using UAC.

Analysis of executables created with the help of this tool revealed that they were being generated programmatically and that an embedded string for the file that gets created was included in each executable, Palo Alto’s Jeff White explains. The filename is also randomized between samples.

White discovered seven total PowerShell scripts that can be generated from the script.

As of late December 2017, Palo Alto has observed 502 unique samples of PowerStager, mainly targeting Western European media and wholesale organizations. A large number of samples, however, were being used for testing and sales proof-of-concepts demonstrations, the researcher says.

White also discovered that certain attributes that PowerStager defines when building the samples can be used to track them. There are also a series of characteristics specific to the generated samples. Although they are usually different between samples, they can prove useful for identification, especially when coupled with said unique obfuscation and PowerShell methods during dynamic analysis.

“While it’s not the most advanced toolset out there, the author has gone through a lot of trouble in attempting to obfuscate and make dynamic detection more difficult. PowerStager has covered a lot of the bases in obfuscation and flexibility well, but it hasn’t seen too much usage as of yet; however, it is on the rise and another tool to keep an eye on as it develops,” White concludes.

 Related: Windows 10 Can Detect PowerShell Attacks: Microsoft

Related: Most External PowerShell Scripts Are Malicious: Symantec

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Ionut Arghire is an international correspondent for SecurityWeek.

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